Heavy-duty tires used on buses and trucks running on expressways, as well as OTR (off-the-road) tires used on earthmovers are usually subjected to severe conditions during service and are constantly placed under high loads. In particular, OTR tires which frequently and violently contact stones and rocks have a great tendency to develop cutting or chipping in their tread portions. The resulting cuts and chips are of course great potential hazards to the driver.
Efforts have long been made to improve the resistance of tires to cutting and chipping, by such methods as the selection of an optimum diene-based polymer from such products as natural rubber and styrene-butadiene rubber (SBR), and the addition of a cyclopentadiene-based petroleum resin as a reinforcing material. It has been disclosed that -a tire having improved resistance to cutting and chipping in the tread portion can be produced from SBR or natural rubber which has incorporated therein a cyclopentadiene-based resin (Japanese Patent Publication No. 38615/73), a cyclopentadiene resin modified with a phenolic resin (Japanese Patent Publication No. 43664/77), or a cyclopentadiene-oxystyrene copolymer (Japanese Patent Publication No. 18938/83). However, none of these tires is completely satisfactory and in view of today's rapid increase in traffic volume and the ever-increasing severity of the conditions in which vehicles are used, tires having an even higher performance are needed.
In order to evaluate the degree of cutting or chipping that develops in the tread portion of a rapidly revolving tire or a tire revolving under a high load resulting from violent contact or abrasion with stones or rocks, the tire must be subjected to practical performance tests including field (on-the-road) tests, simulation tests on miniature tires, and impact cutting tests. However, these tests are highly complicated and involve great cost, labor and time. Fortunately, each of the above-mentioned patents describes a tensile test that can be used to predict the practical performance of tires in the laboratory, and it has been confirmed that the results of this test correlates well with the actual performance of tires in use. The basis of this tensile test is the determination of the elastic energy characteristics of a rubber composition, by measuring its elongation and stress values.
The suitability of this method rests on the fact that the degree of cutting or chipping that develops in the tread portion of a tire depends on how much of the energy generated upon violent contact or abrasion of the tire with stones or rocks can be absorbed by the tread portion.